We were talking to our Microsoft Research colleague Jake Hofman who suggested “why don’t you just find the optimal dates to change the clock by one hour?” So we did. We got the times of sunrise for New York City from here, threw them into R, and optimized.

The result was surprising. The dates of daylight savings time do not come close to minimizing variance in sunrise. If they did, in 2012, DSL would have started on March 25th and ended on September 28th. In actuality, it started on March 11th and ended on November 4th. For NYC, daylight savings time starts too early and ends too late to minimize variance in sunrise. In the heatmap above, the higher the variance, the bluer the squares. The variance minimizing dates are shown in black, and the actual ones in red. The same color coding is used in the plot below, which also shows how the hours would shift if the variance minimizing dates were chosen (see last week’s post for how they actually change).

So what then is the logic behind DSL? We’re not quite sure. There are some leads in this article. We also learned that the US lengthened DSL in 2007 as it believes it that DSL saves energy, but it is not clear that it does.

On the other hand, that foolish change in the start/end of DLS in the USA led to a massive code rewrite for clocks inside other machines — and the immediate obsolescence of clocks and watches with internal ROM code to change from DLS to standard time.

I tested the data for San Jose in California. There is a little variation as compared to NYC. The start time is still March 25th, the end time however comes out to be Sep 30th for 2012 for minimum variance.

One thing that’s clear is if we keep changing our clocks continuously instead of once in an year, we will be able to bring down the variance in the time of sunrise to zero! This was probably the science behind the use of sundial. Every time the sun rises, the sundial would show the same time (based on the shadow) irrespective of what time our modern atomic clocks shows. It would be fun to have a clock that automatically adjusts to morning sunrise everyday. Everyday variations will be small enough to have any effect on our sleep cycle and we would have same sunrise everyday!